30-07-2012, 04:08 PM
Power Flow Control Through Distributed Power Flow Controller By using D-FACTS Concept
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Abstract
In this paper we presents a new component within the
flexible ac-transmission system (FACTS) family, called
distributed power-flow controller (DPFC). The DPFC is derived
from the unified power-flow controller (UPFC). The DPFC can
be considered as a UPFC with an eliminated common dc link.
The active power exchange between the shunt and series
converters, which is through the common dc link in the UPFC, is
now through the transmission lines at the third-harmonic
frequency. The DPFC employs the distributed FACTS (DFACTS)
concept, which is to use multiple small-size single-phase
converters instead of the one large-size three-phase series
converter in the UPFC. The large number of series converters
provides redundancy, thereby increasing the system reliability.
As the D-FACTS converters are single-phase and floating with
respect to the ground, there is no high-voltage isolation required
between the phases. Accordingly, the cost of the DPFC system is
lower than the UPFC.
INTRODUCTION
THE GROWING demand and the aging of networks make
it desirable to control the power flow in power-transmission
systems fast and reliably. The flexible ac-transmission
system(FACTS) that is defined by IEEE as “a powerelectronic
based system and other static equipment that
provide control of one or more ac-transmission system
parameters to enhance controllability and increase powertransfer
capability” , and can be utilized for power-flow
control. Currently, the unified power-flow controller (UPFC)
is the most powerful FACTS device, which can
simultaneously control all the parameters of the system: the
line impedance, the transmission angle, and bus voltage.
DPFC PRINCIPLE
Two approaches are applied to the UPFC to increase
the reliability and to reduce the cost; they are as follows. First,
eliminating the common dc link of the UPFC and second
distributing the series converter. By combining these two
approaches, the new FACTS device—DPFC is achieved. The
DPFC consists of one shunt and several series-connected
converters. The shunt converter is similar as a STATCOM,
while the series converter employs the D-FACTS concept,
which is to use multiple single-phase converters instead of one
large rated converter. Each converter within the DPFC is
independent and has its own dc capacitor to provide the
required dc voltage. The configuration of the DPFC is shown
in Fig. 2. As shown, besides the key components, namely the
shunt and series converters, the DPFC also requires a highpass
filter that is shunt connected on the other side of the
transmission line, and two Y–Δ transformers at each side of
the line.
RESULTS
An experimental setup has been built to verify the principle
and control of the DPFC. One shunt converter and six single
phase series converters are built and tested in a scaled
network. Two isolated buses with phase difference are
connected by the line. Within the experimental setup, the
shunt converter is a single-phase inverter that is connected
between the neutral point of the Y–Δ transformer and the
ground. The inverter is powered by a constant dc-voltage
source. The specifications of the DPFC experimental setup are
listed in the Appendix (see Table I).
Within the setup, multiple series converters are controlled
by a central controller. The central controller gives the
reference voltage signals for all series converters. The
voltages and currents within the setup are measured by an
oscilloscope and processed in computer by using the
MATLAB.
CONCLUSION
This paper has presented a new concept called DPFC. The
DPFC emerges from the UPFC and inherits the control
capability of the UPFC, which is the simultaneous adjustment
of the line impedance, the transmission angle, and the busvoltage
magnitude. The common dc link between the shunt
and series converters, which is used for exchanging active
power in the UPFC, is eliminated. This power is now
transmitted through the transmission line at the third-harmonic
frequency. The series converter of the DPFC employs the DFACTS
concept, which uses multiple small single-phase
converters instead of one large-size converter.